Laminin, a glycoprotein of basement membranes, binds to a specific receptor on the surface of neoplastic and non-neoplastic cells. Laminin exhibits saturatable and compatible binding to the surface of cultured living cells, or to isolated plasma membranes from cells or tissue. The binding coefficient is 2 nM with 50,000 receptors per cell. The receptor was isolated from murine and human carcinomas and melanomas. It has a molecular weight of approximately 67,000 daltons. The laminin receptor purified from human breast carcinoma plasma membranes was used as an antigen to generate monoclonal antibodies (mAbs). Using immunoblotting, the mAbs recognize a single approximately equal to 67,000 dalton protein among all the proteins extracted from breast carcinoma plasma membranes. The mAbs differed in their ability to block binding of laminin to the plasma membrane receptor. Antibody LR1 inhibited virtually 100% of the specific binding of laminin to both the isolated human breast - carcinoma plasma membranes or the living MCF-7 cells. In contrast, antibody LR2 had no effect on laminin binding under identical conditions. Thus, the two types of mAbs may recognize different functional domains on the laminin receptor. Immunoperoxidase staining of human epithelium or cells newly attaching to the basement membrane indicates that the laminin receptors are polarized at the base of the cell.